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Abstract

Alqueva reservoir with a surface area of 250 km2 and total capacity of 4150 hm3 was established in southeast of Portugal a region with scarce availability of water, whether for agricultural or population supply. Since 2006 the water quality of this reservoir is explored using remote sensing techniques. First using MERIS multi-spectral radiometer on-board of ENVISAT-1 and presently with MSI multi-spectral radiometer on-board SENTINEL-2. The existence of two satellites (A and B) equipped with MSI enable the area to be revisited, under the same viewing conditions, every five days. Since 2017 the multidisciplinary project ALOP (ALentejo Observation and Prediction systems) expands the team knowledge about the physical and bio-chemical properties of the reservoir. Previous algorithms from MERIS were tested with the new MSI and improvements were done for water turbidity, concentration of chlorophyll-a and density of cyanobacteria. In addition, new algorithms were developed with MSI for Secchi disk depth (SDD) and diffuse attenuation coefficient (Kd). These algorithms are able to monitor the water quality of all the reservoir with high spatial and temporal resolution enabling to follow, for example, the evolution of a microalgae bloom.Under the same ALOP project, the exchanges of energy, H2O, CO2 and momentum are subject of analysis. One floating platform is equipped with a set of traditional meteorological variables, water temperature at fourteen levels down to the bottom (60 meters), measurements of H2O and CO2 densities and the three components of the wind. The gas densities and the sonic wind integrate an eddy-covariance system for measuring the turbulent fluxes between the reservoir and atmosphere. Alqueva is a Monomictic Lake, most of the year is under thermal stratification and a smaller period, late Autumn and Winter, the lake is fully mixed. Normally the overturn process starts when solar radiation and air temperature decrease causing a decrease of the water surface temperature. These conditions favour vertical mixing forcing surface waters into deeper layers and vice-versa. During stratification the CO2 flux is in average negative (lake uptake) and during mixing period is mostly positive (lake release).